1. Field of the Invention
The present invention relates to a semiconductor light emitting device and a method of manufacturing the same and, more particularly, a semiconductor light emitting device which is employed as a reading/writing light source for a magneto-optic disk device or a light source for a laser printer and a method of manufacturing the same.
2. Description of the Prior Art
As the group III nitride semiconductor laser, the ridge type semiconductor laser which is formed by the steps without dry-etching the active layer and re-growing the crystal of the current constricting layer, etc. and thus can be formed simply are extensively employed.
For example, as disclosed in Patent Application Publication (KOKAI) Hei 4-242985, there is the semiconductor laser which has a GaN compound semiconductor layer as such ridge type group III nitride semiconductor laser.
As the ridge type semiconductor laser, there is the semiconductor laser which has a structure as shown in FIGS. 1A and 1B.
First, in the semiconductor laser shown in FIG. 1A, a buffer 112 made of aluminum nitride (AlN) and a first cladding layer 113 made of n-type aluminum gallium nitrogen (AlGaN) are formed on a sapphire substrate 111 by the MOVPE (metal organic vapor-phase epitaxy) method. Then, a part of a surface of the first cladding layer 113 is covered with a silicon dioxide (SiO2) film (not shown), and then an active layer 114 made of GaP and a second cladding layer 115 made of p-type AlGaN are formed in sequence on a region of the first cladding layer 113, which is not covered with the SiO2 film, by the MOVPE method.
Then, the SiO2 film is removed by hydrofluoric acid, and then another SiO2 film 116 is formed on the second cladding layer 115. An window 116a for electrode connection is formed in the SiO2 film 116 by the photolithography method.
Then, a p-side electrode 117 and an n-side electrode 118 are formed on the second cladding layer 115 exposed from the window 116a and the first cladding layer 113 located on the side of the cladding layer 115 respectively.
With the above steps, a basic structure of the ridge type GaN semiconductor laser diode can be completed.
By the way, the substrate used in the ridge type semiconductor laser is not limited to sapphire, and a silicon carbide (SiC) substrate may be used. An example of such SiC substrate will be explained. with reference to FIG. 1A.
At first, an n-type AlGaN cladding layer 122, an n-type GaN SCH layer 123, an InGaN active layer 124, a p-type GaN SCH layer 125, a p-type AlGaN cladding layer 126, and a p-type GaN contact layer 127 are formed in sequence on an SiC substrate 121 by the MOVPE method.
Then, a stripe-like SiO2 film (not shown) is formed on the contact layer 127, and then the p-type GaN contact layer 127 and the p-type AlGaN cladding layer 126 are selectively removed in sequence by the well-known dry etching method while using the SiO2 film as a mask, whereby the p-type GaN SCH layer 125 is exposed from both sides of the stripe-like SiO2 film.
Then, the SiO2 film is removed and then another SiO2 film 128 is formed. Then, a contact hole 128a is formed on the contact layer 127 by patterning another SiO2 film 128 by using the well-known photolithography method.
Then, a p-side electrode 129 is formed on the contact layer 127 via the contact hole 128a, and also an n-side electrode 130 is formed under the SiC substrate 121.
With the above steps, a basic structure of the ridge type GaN semiconductor laser diode using SiC as the substrate can-be completed.
In this manner, a heat sink effect can be expected. by the semiconductor laser using the SiC substrate rather than the semiconductor laser using the sapphire substrate. Also, since the n-side electrode can be provided on the substrate side, the chip mounting technology as applied to the normal semiconductor laser, etc. can be employed. In addition, since the semiconductor laser using the SiC substrate can have the cleavage property by selecting appropriately the face orientation of the SiC substrate, the Fabry-Perot reflection surface can be formed easily in contrast to the semiconductor laser using the sapphire substrate.
In the semiconductor laser using the group III nitride film compound semiconductor in the prior art, the ridge structure must be employed to form the electrode thereon and also the width of the ridge is restricted by the area of the electrode because of the necessity to assure the alignment margin of the electrode.
There is such a problem that, if the width of the ridge exceeds 2 xcexcm, the optical confinement is weakened in the lateral direction and thus the beam shape is laterally elongated.
A method of performing the optical confinement without the ridge structure or a semiconductor laser in which the current constricting layer is formed is disclosed in Patent Application Publication (KOKAI) Hei 10-294529, Patent Application Publication (KOKAI) Hei 9-232680, and Patent Application Publication (KOKAI). Hei 8-88441.
In Patent Application Publication (KOKAI) Hei 10-294529, an example in which the optical confinement layer is formed on the side of the ridge on the p-type cladding layer and the light is confined by utilizing difference in the refractive index is set forth. It is disclosed to employ InGaN, which has the larger refractive index than the p-type cladding layer, as material of the optical confinement layer. There is such a disadvantage that higher modes are ready to occur if such material having the large refractive index is employed.
In Patent Application Publication (KOKAI) Hei 8-97502, an example in which the current blocking layer is formed in the p-type cladding layer is set forth. The material is InGaN, silicon, etc. This example has a feature to employ the optical absorbing material, but control of the lateral mode is not enoughly performed. In addition, since the photolithography method is employed to form the current path in the current blocking layer, the light emitting portion of the active layer under the current blocking layer is subjected to etching damage if the dry etching is used as the photolithography method, and thus the light emitting characteristic is degraded.
Further, in Patent Application Publication (KOKAI) Hei 9-232680, an example in which the AN, layer is employed as the current constricting layer is set forth, and has a structure to bury both sides of the ridge of the cladding layer by the AlN layer. Such structure cannot help increasing the width of the cladding layer to assure the contact region to the p-side electrode, like the structure shown in FIG. 1B. In addition, the film thickness of the AlN layer is equal to or more than the cladding layer and is thick such as 1 xcexcm. Therefore, the optical confinement is excessively enhanced and thus the higher modes easily occur.
Besides, in Patent Application Publication (KOKAI) Hei 8-88441, an example in which the AlN layer is formed between the p-type cladding layer and the p-type contact layer as the current constricting layer is set forth. However, this example cannot effectively perform the lateral mode control.
It is an object of the present invention to provide a semiconductor light emitting device in which contact to an electrode is set arbitrarily and large and which is ready to control a lateral mode to a desired width, and a semiconductor light emitting device manufacturing method including the step of forming a lateral mode control structure without damage of a current path of an active layer.
According to the present invention, the AlN layer having a thickness of more than 0 nm but less than 300 nm is inserted into the cladding layer, which is formed on or under the active layer made of the group III-V nitride, as the lateral mode controlling layer. The lateral mode controlling layer also acts as the current constricting later.
The AlN layer can reduce difference in the refractive index from the cladding layer in contrast to the AlGaN layer and is difficult to occur the higher modes. In addition, the oscillation in the basic mode can be achieved by setting the thickness of the AlN layer to more than 0 nm, preferably 1 nm, and the oscillation in other modes can be prevented by setting the thickness of the AlN layer to less than 300 nm. Furthermore, if the thickness of the AlN layer is set to less than 300 nm, suppression of the crack generation can be expected.
Since the AlN layer is formed in the p-type or n-type cladding layer, the current can be restricted by forming only the AlN layer close to the active layer without reduction in the thickness of the cladding layer formed on or under the active layer. Therefore, not only the reduction in the threshold current can be achieved but also there is no necessity that the width of the electrode formed over the cladding layer must be narrowed.
Moreover, according to the present invention, the mask is formed on the cladding layer, then the AlN lateral mode controlling layer is formed on the cladding layer and the mask, and then the opening serving as the current path is formed in the AlN lateral mode controlling layer by removing the mask. Therefore, since the active layer is protected by the mask in forming the AlN layer, no damage is caused in the active layer. In addition, since the AlN layer is not subjected to the wet etching, the width of the opening can be controlled not to be expanded excessively.
Furthermore, according to the present invention, since the structure in which the side surface of the opening of the light emitting region formed in the above cladding layer is risen is employed, the light emitting region of the active layer can be protected by the thick cladding layer in forming the lateral mode controlling layer. In addition, since the optical confinement layer is located close to the active layer on both sides of the light emitting region, the good lateral mode control can be achieved and also spreading of the current in the cladding layer can be suppressed to thus reduce the threshold current.
In the present invention, since the high resistance layers are formed under the lateral mode controlling layer, spreading of the current in the cladding layer can be further suppressed and also the threshold current can be further reduced. In addition, since the method of dry-etching the cladding layer is adopted to form the mesa portion in the cladding layer on the active layer, the current constricting effect can be achieved much more.